The present invention relates to a method of charging a fibrous web by wetting it with a wetting agent and then contacting it with an aqueous polar liquid, followed by drying. The invention also pertains to an apparatus that is useful for carrying out the inventive method.
Electrically-charged nonwoven webs are commonly used as filters in respirators to protect the wearer from inhaling airborne contaminants. U.S. Pat. Nos. 4,536,440, 4,807,619, 5,307,796, and 5,804,295 disclose examples of respirators that use these filters. The electric charge enhances the ability of the nonwoven web to capture particles that are suspended in a fluid. The nonwoven web captures the particles as the fluid passes through the web. The nonwoven web typically contains fibers that comprise dielectricxe2x80x94that is, nonconductivexe2x80x94polymers. Electrically-charged dielectric articles are often referred to as xe2x80x9celectretsxe2x80x9d, and a variety of techniques have been developed over the years for producing these products.
Early work relating to electrically-charging polymer foils is described by P. W. Chudleigh in Mechanism of Charge Transfer to a Polymer Surface by a Conducting Liquid Contact, 21 APPL. PHYS. LETT., 547-48 (Dec. 1, 1972), and in Charging of Polymer Foils Using Liquid Contacts, 47 J. APPL. PHYS., 4475-83 (October 1976). Chudleigh""s method involves charging a polyfluoroethylene polymer foil by applying a voltage to the foil. The voltage is applied through a conducting liquid that contacts the foil surface.
An early-known technique for making a polymeric electret in fibrous form is disclosed in U.S. Pat. No. 4,215,682 to Kubic and Davis. In this method, the fibers are bombarded with electrically-charged particles as they issue from a die orifice. The fibers are created using a xe2x80x9cmelt-blowingxe2x80x9d process, where a stream of gas, which is blown at high velocity next to the die orifice, draws out the extruded polymeric material and cools it into a solidified fiber. The bombarded melt-blown fibers accumulate randomly on a collector to create the fibrous electret web. The patent mentions that filtering efficiency can be improved by a factor of two or more when the melt-blown fibers are electrically-charged in this fashion.
Fibrous electret webs also have been produced by charging them with a corona. U.S. Pat. No. 4,588,537 to Klaase et al., for example, shows a fibrous web that is continuously fed into a corona discharge device while positioned adjacent to one major surface of a substantially-closed dielectric foil. The corona is produced from a high-voltage source that is connected to oppositely-charged thin tungsten wires. Another high-voltage technique for imparting an electrostatic charge to a nonwoven web is described in U.S. Pat. No. 4,592,815 to Nakao. In this charging process, the web is brought into tight contact with a smooth-surfaced ground electrode.
Fibrous electret webs also may be produced from polymer films or foils, as described in U.S. Pat. No. Re. 30,782, U.S. Pat. No. Re. 31,285, and U.S. Pat. No. Re. 32,171 to van Turnhout. The polymer films or foils are electrostatically charged before being fibrillated into fibers that are subsequently collected and processed into a nonwoven fibrous filter.
Mechanical approaches too have been used to impart an electric charge to fibers. U.S. Pat. No. 4,798,850 to Brown describes a filter material that contains a mixture of two different crimped synthetic polymer fibers that have been carded into a fleece and then needled to form a felt. The patent describes mixing the fibers well so that they become electrically-charged during the carding. The process disclosed in Brown is commonly referred to as xe2x80x9ctribochargingxe2x80x9d.
Tribocharging also can occur when high-velocity uncharged jets of gases or liquids are passed over the surface of a dielectric film. In U.S. Pat. No. 5,280,406, Coufal et al. disclose that when jets of an uncharged fluid strike the surface of the dielectric film, the surface becomes charged.
A more recent development uses water to impart electric charge to a nonwoven fibrous web (see U.S. Pat. No. 5,496,507 to Angadjivand et al.). Pressurized jets of water or a stream of water droplets are impinged onto a nonwoven web that contains nonconductive microfibers to create the electric charge. The resulting charge provides filtration-enhancing properties. Subjecting the web to an air corona discharge treatment before the hydrocharging operation can further enhance charging.
Adding certain additives to the web has improved the performance of electrets. An oily-mist resistant electret filter media, for example, has been provided by including a fluorochemical additive in melt-blown polypropylene microfibers; see U.S. Pat. Nos. 5,411,576 and 5,472,481 to Jones et al. The fluorochemical additive has a melting point of at least 25xc2x0 C. and a molecular weight of about 500 to 2500.
U.S. Pat. No. 5,908,598 to Rousseau et al. describes a method where an additive is blended with a thermoplastic resin to form a fibrous web. Jets of water or a stream of water droplets are impinged onto the web at a pressure sufficient to provide the web with filtration-enhancing electret charge. The web is subsequently dried. The additives may be (i) a thermally stable organic compound or oligomer, which compound or oligomer contains at least one perfluorinated moiety, (ii) a thermally stable organic triazine compound or oligomer which contains at least one nitrogen atom in addition to those in the triazine group, or (iii) a combination of (i) and (ii).
Other electrets that contain additives are described in U.S. Pat. No. 5,057,710 to Nishiura. The polypropylene electrets disclosed in Nishiura contain at least one stabilizer selected from hindered amines, nitrogen-containing hindered phenols, and metal-containing hindered phenols. The patent discloses that an electret that contains these additives can offer high heat-stability. The electret treatment was carried out by placing the nonwoven fabric sheet between a needle-like electrode and an earth electrode. U.S. Pat. Nos. 4,652,282 and 4,789,504 to Ohmori et al. describe incorporating a fatty acid metal salt in an insulating polymer to maintain high dust-removing performance over a long period of time. Japanese Patent Kokoku JP60-947 describes electrets that comprise poly 4-methyl-1-pentene and at least one compound selected from (a) a compound containing a phenol hydroxy group, (b) a higher aliphatic carboxylic acid and its metal salts, (c) a thiocarboxylate compound, (d) a phosphorous compound, and (e) an ester compound. The patent indicates that the electrets have long-term storage stability.
A recently-published U.S. patent discloses that filter webs can be produced without deliberately post-charging or electrizing the fibers or the fiber webs (see U.S. Pat. No. 5,780,153 to Chou et al.). The fibers are made from a copolymer that comprises: a copolymer of ethylene, 5 to 25 weight percent of (meth)acrylic acid, and optionally, though less preferably, up to 40 weight percent of an alkyl (meth)acrylate whose alkyl groups have from 1 to 8 carbon atoms. Five to 70% of the acid groups are neutralized with a metal ion, particularly an ion of zinc, sodium, lithium, or magnesium, or a mixture of these. The copolymer has a melt index of 5 to 1000 grams (g) per 10 minutes. The remainder may be a polyolefin such as polypropylene or polyethylene. The fibers may be produced through a melt-blowing process and may be cooled quickly with water to prevent excess bonding. The patent discloses that the fibers have high static retention of any existing or deliberate, specifically induced, static charge.
The present invention provides a new method for making a fibrous electret web. In brief summary, the method comprises: wetting a fibrous web, which web comprises nonconductive fibers, with a wetting agent; saturating the wetted web in an aqueous polar liquid; and substantially drying the web. The fibrous web may be a woven web or a nonwoven web, and it may be used as a filter element in a finished article such as a respirator or filter cartridge.
The present method differs from known charging methods in that the web is wetted with a wetting agent before being saturated with an aqueous polar liquid. The inventors discovered that the wetting step is beneficial in that it can allow a better performing filter to be provided as measured by the Quality Factor parameter described below. The wetting step may increase the measured charge density of the fibrous web and thus enable the better performance to be obtained.
As used in this document:
xe2x80x9caqueousxe2x80x9d means that the aqueous polar liquid contains at least about 10% water by volume.
xe2x80x9celectric chargexe2x80x9d means that there is charge separation.
xe2x80x9cfibrousxe2x80x9d means possessing fibers and possibly other ingredients.
xe2x80x9cfibrous electret webxe2x80x9d means a web that contains fibers and that exhibits a quasi-permanent electric charge.
xe2x80x9cliquidxe2x80x9d means the state of matter between a solid and a gas.
xe2x80x9cnonconductivexe2x80x9d means possessing a volume resistivity of about 1014 ohmxc2x7cm or greater at room temperature (22xc2x0 C.).
xe2x80x9cnonwovenxe2x80x9d means a structure or portion of a structure in which fibers are held together by a means other than weaving.
xe2x80x9cpolar liquidxe2x80x9d means a liquid that has a dipole moment of at least about 0.5 Debye and that has a dielectric constant of at least about 10.
xe2x80x9cpolymerxe2x80x9d means an organic material that contains repeating linked molecular units or groups, regularly or irregularly arranged.
xe2x80x9cpolymericxe2x80x9d means containing a polymer and optionally other ingredients.
xe2x80x9cpolymeric fiber-forming materialxe2x80x9d means a composition that contains a polymer, or that contains monomers capable of producing a polymer, and possibly contains other ingredients, and that is capable of being formed into solid fibers.
xe2x80x9cquasi-permanentxe2x80x9d means that the electric charge resides in the web under standard atmospheric conditions (22xc2x0 C., 101,300 Pascals atmospheric pressure, and 50% humidity) for a time period long enough to be significantly measurable.
xe2x80x9csaturatingxe2x80x9d means wetting the web with the maximum, or substantially the maximum, amount possible of a liquid.
xe2x80x9cwebxe2x80x9d means a structure that is significantly larger in two dimensions than in a third and that is air permeable.
xe2x80x9cwettingxe2x80x9d means contacting or coating substantially all the surface area of the web that is desired to be wetted.
xe2x80x9cwetting liquidxe2x80x9d means a liquid that meets the Wetting Test described below and that dissolves in the aqueous liquid that is used to saturate the web.